How was the Glenrowan Siege portrayed in Justin Kurzel’s adaptation of Peter Carey’s novel, and how does it compare to what we know of the history?

How was the Glenrowan Siege portrayed in Justin Kurzel’s adaptation of Peter Carey’s novel, and how does it compare to what we know of the history?
The core of the Glenrowan story is Ned Kelly. Everything that occurs is either directly or indirectly linked to him and his decisions. Naturally this should position him as the protagonist of the story, though protagonist usually implies that character is the “good guy”. As I’ve discovered, simplistic terms like “good”, “bad”, “hero”, or “villain” are just completely inadequate to describe someone as complex as Edward Kelly.
Where did the idea for the novel originate? What makes the story of the Glenrowan siege so compelling?